The goal of this project is to further our understanding of the descending, efferent neural system. We present a new method of efferent activation by localized electrical stimulation of the inferior colliculus (IC). The IC is grossly divided into three subnuclei, the central (ICC), external (ICX) and dorsal (ICD). The descending projections to the IC derive from a variety of locations within the central nervous system. The inputs to the ICX derive from auditory as well as nonauditory areas. The input to the ICD is from auditory neural centers. The ICC receives no descending projections but sends efferent projections to lower brainstem locations, as do ICX and ICD. The divergent inputs to and outputs from the IC suggest possible different roles or functions for each IC descending system. The preliminary studies show that ICC stimulation has frequency specific affects on the neural output of the cochlea. The Specific Aims describe studies that examine the effects of electrical stimulation of the individual subnuclei of the contralateral and ipsilateral IC. Aim I hypothesizes that localized stimulation of the ICX results in diffuse reductions of neural output of the cochlea. Such a generalized reduction in cochlear output might form the basis of the phenomenon of selective attention to another sensory event. Aim I also hypothesizes that localized stimulation of the ICC and ICD results in tonotopically localized effects on the cochlear responses. The cochlear whole-nerve action potential and the ear canal otoacoustic emission responses will be measured with and without IC stimulation. The two response measures allow potential differentiation of lateral and medial efferent systems. Medial efferent activation affects the transduction process through its influence on the outer hair cell. The lateral system only affects the afferent synapse below the inner hair cell. Aim II hypothesizes that localized IC stimulation will result in frequency specific increases in cochlear neural output under conditions of masking noise. These studies will focus on the efferent role in the improvement of signal detection in noise. Compared to previous methods of efferent activation, IC stimulation may mimic more closely the role of the efferent system in the awake behaving animal. These studies have great potential to improve our understanding of the efferent system and its role in auditory processing.